Ionized gas or “plasma” may be used during processing and fabrication of semiconductor devices, flat panel displays and in other industries requiring etching or deposition of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit (IC) wafers, or to sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in a manufacturing or fabrication processes is typically done by introducing a low pressure process gas into a plasma chamber surrounding a work piece (substrate), such as an IC wafer. The molecules of the low-pressure gas in the chamber are ionized by a radio frequency (RF) energy (power) source to form a plasma which flows over the substrate. The plasma chamber is used to maintain the low pressures required for the plasma and to serve as a structure for attachment of one or more electrodes which serve as RF energy sources.
Plasma may be created from a low-pressure process gas by inducing an electron flow which ionizes individual gas molecules by transferring kinetic energy through individual electron-gas molecule collisions. Typically, electrons are accelerated in an electric field such as one produced by RF power. This RF power may have a low frequency (below 550 KHz), high frequency (13.56 MHz), or a microwave frequency (2.45 GHz).
Etching may be performed by plasma etching or reactive ion etching (RIE). A plasma etching system may include a single RF power source, or a plurality of such sources operating at one or more frequencies with a corresponding number of electrodes, at least one of which is located within the process chamber. A plasma is generated adjacent the substrate, the latter typically being co-planar with the electrode and supported by a substrate support member within the process chamber. The RF energy may be coupled to the plasma by capacitive means, by inductive means, or by both capacitive and inductive means. The chemical species in the plasma are determined by the source gas(es) used.
Plasma etching methods and apparatus using capacitive coupling are generally illustrated in U.S. Pat. Nos. Re. 30,505 and 4,383,885. A capacitively coupled plasma etching apparatus typically includes a lower electrode on which a semiconductor substrate or wafer is placed and an upper electrode opposing the lower electrode. The lower and upper electrodes are connected to respective RF power supplies. The upper electrode may be divided into a plurality of segments, each of which may be excited by a dedicated RF power supply. The RF power ionizes the source gas(es) and thereby produces a plasma. A range of radio frequencies may be used.
A method and apparatus using inductive coupling for obtaining a plasma for processing of IC wafers is described in U.S. Pat. No. 4,948,458. The plasma etching apparatus described includes an enclosure having an interior bounded at least in part by a RF transparent window. A planar coil is disposed proximate the window, and a RF energy source is coupled through an impedance matching circuit to the coil. The planar coil radiates the RF energy such that a planar magnetic field is induced in the interior of the enclosure. In alternate configurations, as, for example, in U.S. Pat. No. 5,234,529, the magnetic field is produced by a solenoidal coil that surrounds at least a part of a process chamber with walls that permit the magnetic field to pass into the process chamber. The magnetic field produces an electric field in accordance with Faraday's law. A plasma is generated thereby from the process gas. This plasma reacts with the surface of the semiconductor wafer, etching away material from the surface.
A plasma may also be used in chemical vapor deposition (CVD) to form thin films of metals, semiconductors or insulators (or, conducting, semiconducting or insulating materials) on a semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply the required reaction energy for deposition of the desired materials. Typically, RF energy is used to produce this plasma.
Unfortunately, it is difficult to quickly assess the quality of the etching or deposition process in plasma processing. Presently, one of the main methods used to assess processing quality involves the rather tedious and costly steps of processing a wafer in the reactor, removing the wafer, and then examining the wafer to obtain the data necessary to determine the quality of a given plasma process. Furthermore, changes in the process due to equipment malfunctions, such as defects in mass flow controllers, almost always reduce process yields, and cannot generally be corrected until after test wafers have been processed and examined, which is a time consuming and expensive proposition.
It would be beneficial, therefore, to have an in situ monitoring system capable of providing information about the quality of the plasma processing in a plasma reactor system.
Optical emission spectroscopy is a method currently used to detect a process endpoint in plasma etching systems. This technique is in situ and is possible because the plasma excites certain atomic and molecular species present in the plasma and causes them to emit light of wavelengths that are characteristic of the species being etched.
In an optical monitoring system for performing optical emission spectroscopy, specific wavelengths of the light emitted from the plasma are selected and fed to detectors, such as photodiodes, photomultipliers, and array detectors, which convert the light intensities into electrical signals. It is known that the intensity of the detected raw signals is related to the level of light detected. By selecting wavelengths which correlate to the reaction products of the particular process, the process may be monitored either at specific wavelengths or at all wavelengths by a spectral scan. In particular, by selecting a wavelength which corresponds to the emissions generated by the layer below the layer which is being etched, the point at which that layer is reached may be easily detected. When the film being etched has been completely removed from the underlying material or film, there is a change in the chemical composition of both the gas phase and the remaining film. Product species from the etched film are no longer being generated, and the concentration of some reactants increases because they are no longer being consumed by the reaction. These chemical changes show up as changes in optical emission intensities. Thus, by continuously monitoring the intensity of an appropriate emission feature (i.e., either a reactant consumed in or a product of the etch reaction), a change in emission intensity generally signals removal of the film being etched and contact of the etching agent with the underlying material or film. This change signals the process endpoint and may either be the result of an increase in reactant emission, a decrease in product emission or the presence of another reactant emission.
A patent which discloses an apparatus and method for performing plasma characterization in a plasma process is U.S. Pat. No. 5,691,642 (the '642 patent), the disclosure of which is incorporated herein by reference. Specifically, the '642 patent describes a method and apparatus for accurately characterizing the electron density and distribution of a confined plasma on the basis of high-frequency, broadband electromagnetic measurements. The technique involves noninvasive, broadband measurement of electromagnetic transmission through the plasma. In one implementation, multivariate analysis techniques are employed to correlate features of the resultant spectra with plasma characteristics such as electron density or electron distribution. Alternately, such techniques are used to correlate the resultant spectra with parameters relating to conditions under which the plasma is generated. More specifically, the quantitative plasma characterization technique involves generating a set of broadband calibration spectra by measuring transmission of electromagnetic energy through a calibration plasma. Each broadband calibration spectrum is obtained using a different set of reference parameters being related to predefined quantitative characteristics. The reference parameters may comprise known values of quantitative characteristics of the calibration plasma including, for example, electron distribution or electron density. Alternately, the reference parameters may comprise known values of operating conditions within the chamber bearing a predetermined empirical relationship to particular plasma quantitative characteristics. A reference parameter transformation, which relates measured spectra of electromagnetic energy transmitted through the calibration plasma to values of the reference parameters, is derived on the basis of the broadband calibration spectra. A test spectrum is then obtained by measuring transmission of electromagnetic energy through the subject plasma. Values of the predefined quantitative characteristics of the subject plasma are then determined by analyzing the test spectrum using the reference parameter transformation.
However, a major shortcoming of the invention disclosed in the '642 patent is the need to measure the transmission of electromagnetic energy at microwave wavelengths through the plasma, which is a relatively complex process. Another shortcoming is that the disclosed method and apparatus do not provide information about variation in the plasma across the wafer, so that corresponding cross-wafer variations in plasma processing are not detectable.
U.S. Pat. No. 5,200,023 describes an infrared television camera that monitors the etching of a substrate in-situ in an etch chamber. Temporal and spatial resolutions of IR emissions are obtained by monitoring the top surface of the substrate in two-dimensions throughout the course of the etching process. Anomalies in temperature detected on the top surface of the substrate can indicate defects in the substrate itself or in the operation of the etching apparatus. Process feedback control is achieved by adjusting various parameters of the etching apparatus (i.e., gas-pressure, flow pattern, magnetic field, coolant flow to electrode, or the like) to compensate for etching anomalies. Etch uniformity and etch endpoint monitoring is achieved by monitoring the IR emissions resulting from exothermic reaction of the film being etched. IR emissions decline at the end of an exothermic etch reaction. Particulate matter suspended in the plasma, which might harm the substrate, can be identified with a second IR television camera, which images the region above the substrate. Particulate matter appears as localized “hot spots” within the gas plasma, and the identification of particulate matter allows corrective measures to be taken. However, the invention described in U.S. Pat. No. 5,200,023 detects just IR radiation, which is only a limited part of the spectrum of light emitted by the plasma. This limits the amount of information about the across-wafer variations in the plasma processing that can be obtained.